1. Field of the Invention
The present invention relates to an electrophotographic photosensitive member in which functions are separated, and in particular to an improvement in a charge generation layer for improving electrophotographic characteristics.
2. Related Background Art
There has recently been proposed an electrophotographic photosensitive member in which the photosensitive layer is separated in functions as a laminar structure of a charge generation layer and a charge transport layer. Various improvements have been made, on such photosensitive member, in sensitivity to visible light, charge retaining power, surface strength etc. as disclosed for example in the U.S. Pat. Nos. 3,837,851 and 3,871,882.
Such photosensitive member with separated functions is composed at least of a charge generation layer and a charge transport layer. Charge carriers generated by light absorption in the charge generation layer are injected into the charge transport layer and move to the surface to neutralize the surface charge of the photosensitive member, thereby generating electrostatic contrast.
In the above-described process, the charge generation layer plays an extremely important role. More specifically, electrophotographic characteristics such as uniform and abundant generation of charge carriers, effective injection of thus generated charge carriers into the charge transport layer and method of smooth dissipation of opposite charge carriers to the support principally rely on the charge generation layer. The charge generation layer is essentially composed of a binder and an organic pigment which is a charge generating material, and the weight ratio of the binder to the organic pigment is generally as high as 25 to 100 wt.%. Consequently the binder has an extremely important effect on the movement of charge carriers generated in the charge generation layer, and the basic structure, functional groups, molecular weight, purity etc. of the binder are deeply related with the electrophotographic characteristics of the photosensitive member such as the sensitivity, charge potential, durability etc.
However, in prior references and patents, the binder in the charge generation layer has been regarded as an auxiliary material for the organic pigment, which is the charge generating material, for simply providing dispersibility and adhesion.
For this reason, conventional function-separated electrophotographic photosensitive member has been associated with various defects in potential characteristics such as retentive potential, potential functuation, photomemory (an undesirable effect of an image to an immediately following image in case of continuous image formation). Also the sensitivity is not sufficient.
The present inventors have understood the binder as another principal electronic material in the charge generation layer, and have reached the present invention through the understanding of the binder from its molecular aspect, such as structure, molecular weight, purity etc.